1. Field of the Invention
The present invention relates to a safety valve for a gastight battery and a gastight battery using the safety valve, and more particularly to a safety valve having a valve element function which is communicated with the outside when a pressure in the battery becomes excessively large.
2. Description of the Related Art
With respect to a gastight battery, when an abnormal reaction occurs in the battery for some reasons so that a pressure in the battery is elevated, there exists a possibility that the gastight battery causes damages on a human body or objects due to the explosion of the battery per se. To avoid such a possibility, it is necessary to release a gas in the battery before the explosion occurs. A safety valve is provided to a battery case for this end.
The safety valve has a valve element which is broken and is opened with a predesigned internal pressure so as to release an internal gas to the outside. Although a break-open pressure is higher than a normal battery internal pressure in terms of required properties of the safety valve as a matter of course, it is desirable that the release pressure (operation pressure) is as low as possible in terms of safety.
Further, in the case where an amount of gas generated in the battery is large, when an area of the valve element through which the gas is released is small, the battery internal pressure is temporarily increased thus giving rise to a possibility that this increase of the battery internal pressure leads to the explosion.
Accordingly, it is important that a gas releasing portion which is formed by breaking an easy-to-break groove has an area sufficient for releasing an amount of gas generated in the battery.
Further, the gastight battery is used for a mobile phone, a digital camera or the like and hence, depending on the manner of handling such a device, there exists a possibility that a user falls the gastight battery on a ground or a floor so that the valve element portion is broken resulting in leakage of an electrolysis solution or the like. Accordingly, it is important that the valve element is not easily damaged when such a fall accident occurs so that the valve element is required to exhibit a high falling strength.
To satisfy the above-mentioned object, the gastight battery has a safety valve as a safety mechanism. As a method of forming the safety valve, there has been known a method in which a thin film having a predetermined size is formed in a sealing plate surface by pressing, etching or the like (a press valve or an imprinting valve).
Further, there has also been known a method in which a opened hole having a predetermined size is formed in a sealing plate surface, and a thin film is adhered to the opened hole as a valve element by cladding or welding (clad valve).
The clad valve has advantages such as a low operation pressure, a slightly larger release area and slightly larger fall strength which are required properties of a safety valve. However, the clad valve has a disadvantage that manufacturing steps of the clad valve require a pressing step and a cladding step so that the manufacturing steps are complicated and a manufacturing technique is difficult whereby a yield is low thus pushing up a cost.
To the contrary, in the manufacture of a press valve, a valve element can be formed in an inner surface of a sealing plate only by pressing so that the press valve can be manufactured at a low cost.
Further, the press valve can easily ensure a safety valve area of a predetermined size or more sufficient for releasing a gas. However, it is difficult for the press valve to acquire a stable low operation pressure compared to a clad valve.
Under such circumstances, the development of the press valve has been in progress along a precise bottom-dead-center control of a press device or the improvement and development of a proper material.
JP-A-11-273640 discloses a safety valve in which a dome-shaped thin-wall valve element is formed on a sealing plate, and an easy-to-break groove is formed in the vicinity of the periphery of the valve element.
JP-A-2005-135873 discloses a safety valve in which a thin-wall valve element is formed on a sealing plate, at least one dome portion which projects toward the inside of a battery is formed on the valve element, and a breaking groove which eases breaking of the valve element is formed on at least one peripheral mountain of the dome portion.